This invention relates to a glow plug for diesel engines of motor vehicles, comprising a hollow metal body, a current feeder and a tip-closed tubular metal sheath, containing an electrical spiral embedded within an electrically insulated material. The invention also relates to a process of manufacture of the spiral.
As well known, a winding arranged within a sheath performs the function of causing the sheath to glow for sufficiently heating the combustion chamber in which the sheath projects in order to facilitate the start of the heat engine at low temperatures. However, the winding requires some time for raising the sheath temperature to the required value for fuel self-combustion. This waiting time, commonly referred to as preheating time, is signalled to the driver by means of an alarm lamp which is normally lit at the first rotation of the igniting change-over switch and is extinguished when the engine is ready for start. However, after this instant, the winding continues to be supplied for an additional time to enable the driver to provide for start operation, whereupon the supply is discontinued.
In order to reduce said preheating time to a few seconds and also avoid that during said additional supply time the winding is unduly heated, with the risk of filament breakage, several expedients have been made to the winding shape and structure.
According to the British Pat. Nos. 1,127,454 and 2,013,277, in order to provide a faster heating of the sheath at supply beginning and then a current limitation to avoid any overheating of the filament, the latter comprises two series connected spirals having different operation and characteristics. One of these spirals, particularly the end spiral, performs the function of particularly heating the plug tip (heating spiral), while the other spiral, or initial spiral, performs the function of current control (resisting spiral), so that during the additional supply time the filament temperature is maintained within reasonable limits.
It is evident that the manufacture of such a glow plug is highly costly both for the use of the two spirals or windings having different characteristics and the required welding operation for the connection thereof.
Also, it is event that, in order to provide a good welding operation between the two spirals or windings, it is required that during operation the two ends to be connected are brought to and maintained correctly juxtaposed, and additionally that said ends terminate at the same level, or that none of these ends project relative to the other. Of course, in order to satisfy these conditions, use should be made of a sophisticated and accordingly expensive equipment, to the disadvantage of the final cost of the glow plug.
It should also be noted that the filaments for the two spirals or windings have a small diameter, and accordingly the connection of the ends thereof requires the use of a laser apparatus, since an actual microweld has to be effected. Further, in order to carry out the various operations, an operator needs a monitor or an enlarging apparatus.
In any case, weld would generate a discontinuity in the filament, that is a weak location where an interruption may occur either due to mechanical failure or fusion due to undue heating of a small size zone.
Finally, the use of two spirals or windings involves complications in construction when desiring to pass from a production of glow plug having certain characteristics to a further production of glow plugs having different characteristics. In this case, it would be necessary to replace one or both spirals or windings.